A number of processes have been proposed for the manufacture of submicron carbides, nitrides, borides and other compounds by gas phase reactions at elevated temperatures. The reactants are usually heated by plasma arc techniques in which a gas is heated as it is passed though an arc.
In U.S. Pat. No. 3,232,706 a process of making submicron carbides and nitrides by vaporizing at least one solid reactant in a furnace created between two non-consumable electrodes is disclosed. The particulate product is swept from the furnace by a sweep gas.
In U.S. Pat. No. 3,485,586 a process of making submicron titanium carbide is described in which a titanium halide in vapor state is reacted with a source of carbon, halogen and hydrogen. Hydrogen is heated by passing through an electric arc and the titanium chloride and the source of carbon, etc. are fed into the hot hydrogen stream which is simply passed into a settling chamber and then to a bag filter, electrostatic precipitator or other means for separation of powders from gasses.
In U.S. Pat. No. 3,992,193 a process of making metal powder by direct reduction in an arc heater is described. A suggested cooling method is quenching the already formed droplets of metal by passing through a nozzle.
In U.S. Pat. Nos. 4,022,872 a process of preparing finely-divided borides, carbides, silicides, nitrides and sulfides of metals and metalloides by reaction in the gas phase of a vaporous halide of the metal or metalloid and a reactant gas containing a source of the non-metallic element with which the metal or metalloid is to be reacted is disclosed. Heating is by hydrogen plasma arc. The reactants are mixed with the heated gas stream along with a cooler inert gas to cancel the swirling action of the plasma arc.
In U.S. Pat. No. 3,666,408 a process for the production of oxides of nitrogen is disclosed in which the nitrogen and oxygen are reacted in a plasma created by electric arc discharge. The resultant gaseous reaction mixture is cooled to below the dissociation temperature by expanding the plasma through a nozzle and contacting the expanded reaction mixture with cooler recycle gas.
In U.S. Pat. No. 4,146,389 a process for thermal reduction of alumina to aluminum in which the reactants are heated in a dispersed electrical discharge reactor is described.
No prior art process of which the applicant is aware teaches heating reaction mixtures at plasma temperatures followed by condensation of solid reaction products from the mixture by cooling in an expansion nozzle. In the process wherein nitric oxide is fixed from air or nitrogen and oxygen mixtures the product is gaseous. Moreover, the reaction has taken place before the mixture enters the nozzle. Also, nitric oxide has an anomalous behavior enabling it to cool vibration modes of molecular energy storage about 1000 times after than most other diatomic molecules. It is necessary to cool vibrational modes to arrest undesired reactions. Hence, it cannot be expected that nozzle cooling will be effective in cooling reaction mixtures to avoid back reactions for all conceivable processes.
In the applicant's process disclosed herein chemical reaction, nucleation, and condensation of solid particles all take place in the cooling nozzle and in a way to avoid undesired reactions. An essential aspect of the applicant's process is providing in the reaction mixture a low molecular weight gas that has the ability to enable transfer of energy from vibrational modes of molecular energy storage. Another essential aspect of the applicant's process is the provision of an inert gas in the reaction mixture that, while at the temperature of the remainder of the mixture passed into the nozzle, provides a sink for exotheraic condensation reactions and the latent heat of condensation in the nozzle.